This invention relates to engine installations for gas turbine powered aircraft. More particularly this invention relates to an engine mounting arrangement wherein the weight and outer diameter of the engine installation is minimized by configuring the engine cowling to bear structural loading and mounting components of the propulsion system to the cowling rather than to the gas turbine engine.
In prior art engine installations wherein a propulsion system including a gas turbine engine is mounted beneath the wing of an aircraft (or depends from other aircraft structure), the engine is generally mounted to the aircraft by means of a pylon or other structural member. The remaining propulsion units of the engine installation are then structurally connected to the engine. For example, in such an engine installation, an air inlet duct is generally bolted to the forward face of the gas turbine engine and an exhaust system, including apparatus such as an exhaust nozzle and thrust reversers, is bolted to the rear face of the engine. A sheet metal cowling assembly is then installed to enclose or cover the propulsion system and form an aerodynamically smooth surface.
Since an engine installation configured in this manner includes components such as the air inlet, exhaust nozzle and thrust reversers which are connected directly to the engine and extend axially therefrom, static and dynamic load forces are exerted on the engine. For example, because these components generally have substantial axial length and substantial mass, they are subjected to inertial forces which cause substantial bending moments to be exerted on the gas turbine engine. Further, during aircraft flight, pressure differences established by airflow around and through the air inlet and the exhaust system cause additional bending moments to be exerted on the gas turbine engine.
To prevent such bending moments from being exerted on internal engine structure such as the shafts which link the engine compressor and turbine stages, the load in prior art engine arrangements must generally be borne by the engine casing or shell that surrounds the internal engine structure. Thus, in the past, the engine casing and/or internal engine structure has necessarily been constructed to withstand substantial structural loading. Configuring an engine to meet these load requirements has meant that the engine casing and various other engine components have been necessarily constructed of relatively heavy material, with the various components having a greater cross-sectional geometry than would be necessary if the engine were not subjected to structural loading by other components of the propulsion system.
In view of the above, it can be recognized that the arrangement of the prior art engine installation results in two major problems and drawbacks, each of which can substantially affect aircraft performance and efficiency. First, a substantial weight penalty can be incurred, since a gas turbine engine constructed to withstand the aforementioned structural loading is heavier than required from the standpoint of engine performance. Secondly, as described above, gas turbine engines configured to withstand structural loading induced by other components of the engine installation are larger in diameter than if the engine were configured solely on the basis of propulsion requirements. Due to this larger engine diameter, the outer diameter of the engine installation is greater than would otherwise be necessary. Thus, even though the engine cowling is aerodynamically contoured, significant drag can be encountered because of the relatively large cross-sectional geometry and the relatively large surface area of the engine cowling.
It will be recognized that the aforementioned considerations, although pertaining to all engine installations of the type described, become especially important in supersonic aircraft. In particular, due to relatively complex air supply requirements and exhaust flow requirements, the air inlets, exhaust nozzles and thrust reversers of supersonic engine installations are relatively long and heavy and hence exert strong bending moments on the engines of the prior art arrangement. Further, at transonic and supersonic speeds, drag considerations and flow separation along the aerodynamically contoured surface of the engine installation become especially important.
In addition to weight and drag considerations, it is both desirable and necessary to configure an engine installation such that the gas turbine engine is accessible for maintenance and engine removal procedures. In this respect, the engine cowlings of prior art engine installations often consist of removable sheet metal panels arranged such that all or part of the cowling can be removed for engine replacement operations or for access to particular engine and propulsion unit components. In other engine mounting arrangements wherein the gas turbine engine is effectively enclosed by aircraft structure, i.e., mounted within the aircraft tail assembly or within the wings, either a cowling or a portion of the aircraft structure is configured for removal or folding movement relative to remaining portions of the engine mounting arrangement to thereby expose the engine. In many of these arrangements, weight penalties are incurred, undue time is required for engine removal, and inspection, repair and maintenance are difficult with the gas turbine engine in place.
With respect to engine removal and installation, further problems are encountered with many of the prior art engine installations. Specifically, in many aircraft configurations, the engines are located a considerable height above the level of the ground and specially designed ground-support equipment is required for removing the engine and/or positioning the engine during engine installation procedures. Although such ground support equipment may perform satisfactorily, it is often complex and costly. Further, such specialized equipment may not available at each location where it is necessary to remove an aircraft engine.
Accordingly, it is an object of this invention to provide a lightweight engine installation wherein minimal structural loading is borne by the gas turbine engine.
It is another object of this invention to provide an aircraft engine installation of minimal cross-sectional geometry to thereby reduce drag forces exerted on the engine installation during aircraft flight.
It is yet another object of this invention to provide an engine-cowling arrangement for use in an engine installation of a gas turbine powered aircraft wherein loading caused by components of the engine installation such as air inlets and exhaust nozzles are not coupled to the gas turbine engine, but are efficiently borne by a cowling structure which surrounds the gas turbine engine.
It is still another object of this invention to provide an engine mounting arrangement of the above-described type wherein the engine cowling is arranged to provide ready access to the gas turbine engine for purposes of inspection, repair and removal.
Even further it is an object of this invention to provide an engine mounting arrangement of the above-described type including means for raising and lowering the gas turbine engine between the cowling and ground level during engine removal and installation procedures.